The present invention relates in general to external-cavity optically-pumped semiconductor lasers (hereinafter, OPS-lasers) including a surface-emitting, semiconductor multilayer (periodic) gain-structure. The invention relates in particular to arrangements of such lasers which can provide fundamental laser output-power of about two Watts (2.0 W) or greater, and intracavity frequency-converted arrangements of such lasers which can provide harmonic laser output-power of about one-hundred milliwatts (100 mW) or greater.
The term OPS-lasers, as used herein, refers to a class of vertical-cavity surface-emitting semiconductor lasers wherein optical gain is provided by recombination of electrical carriers in very thin layers, for example, about 150 {dot over (A)}ngstrom units (xc3x85) or less, of a semiconductor material. These layers are generally termed quantum-well (QW) layers or active layers.
In an OPS-laser, several QW layers, for example, about fifteen, are spaced apart by separator layers also of a semiconductor material, but having a higher conduction band energy that the QW layers. This combination of active layers and separator layers may be defined as the gain-structure of the OPS-laser. The layers of the gain-structure are epitaxially grown. On the gain-structure is an epitaxially-grown multilayer mirror-structure, often referred to as a Bragg mirror. The combination of mirror-structure and gain-structure is referred to hereinafter as an OPS-structure.
In an (external cavity) OPS-laser, another (conventional) mirror, serving as an output-coupling mirror is spaced-apart from the OPS-structure, thereby forming a resonant cavity with the mirror-structure of the OPS-structure. The resonant cavity, accordingly, includes the gain-structure of the OPS-structure. The mirror-structure and gain-structure are arranged such that QW layers of the gain-structure are spaced apart by one half-wavelength of the fundamental laser wavelength, and correspond in position with antinodes of a standing-wave of the fundamental laser-radiation in the resonator. The fundamental-wavelength is characteristic of the composition of the QW layers.
Optical pump-radiation (pump-light) is directed into the gain-structure of the OPS-structure and is absorbed by the separator layers of the gain-structure, thereby generating electrical-carriers. The electrical-carriers are trapped in the QW layers of the gain-structure and recombine. Recombination of the electrical-carriers in the QW layers yields electromagnetic radiation of the fundamental-wavelength. This radiation circulates in the resonator and is amplified by the gain-structure thereby generating laser-radiation.
OPS-lasers have often been used in the prior art as a means of conveniently testing QW structures for later use in electrically-pumped semiconductor lasers. More recently, OPS-lasers have been investigated as laser-radiation sources in their own right. The emphasis of such investigation, however, appears to be on providing a compact, even monolithic, device in keeping with the generally compact nature of semiconductor lasers and packaged arrays thereof.
The gain-structure of OPS-structures may be formed from the same wide range of semiconductor-materials/substrate combinations contemplated for diode-lasers. These include, but are not limited to, InGaAsP/InP InGaAs/GaAs, AlGaAs/GaAs, InGaAsP/GaAs and InGaN/Al2O3, which provide relatively-broad spectra of fundamental-wavelengths in ranges, respectively, of about 960 to 1800 nanometers (nm); 850 to 1100 nm; 700 to 850 nm; 620 to 700 nm; and 425 to 550 nm. There is, of course, some overlap in the ranges. Frequency-multiplication of these fundamental-wavelengths, to the extent that it is practical, could thus provide relatively-broad spectra of radiation ranging from the yellow-green portion of the electromagnetic spectrum well into the ultraviolet portion.
In conventional solid-state lasers, fundamental-wavelengths, and, accordingly, harmonics thereof (produced by frequency-doubling or frequency-mixing) are limited to certain fixed wavelengths characteristic of a particular dopant in a particular crystalline or glassy host, for example, the well-known 1064 nm wavelength of neodymium-doped yttrium aluminum garnet (Nd:YAG). While one of these characteristic wavelengths may be adequate for a particular application, it may not be the optimum wavelength for that application.
OPS-lasers provide a means of generating wavelengths, in a true CW mode of operation, which can closely match the optimum wavelength for many laser applications, in fields such as medicine, optical metrology, optical lithography, and precision laser machining. Prior-art OPS-lasers, however, fall far short of providing adequate power for such applications. It is believed that the highest fundamental output-power that has been reported, to date, for an OPS-laser is 700 mW at a wavelength of about 1000 nm (Kuznetsov, et al., IEEE Photonics Tech. Lett 9, 1063 (1997)). For an intracavity frequency-doubled OPS-laser, it is believed that highest output-power that has been reported is 6 mW at a wavelength of about 488 nm (Alford et al. Technical Digest of the IEEE/OSA Conference on Advanced Solid State Lasers, Boston Mass., Feb. 1-3 1999, pp 182-184). It believed that there has been no report to date of generation of continuous wave (CW) ultraviolet (UV) radiation in an OPS-laser, either directly or by frequency-multiplication.
However flexible an OPS-laser may be in potentially offering a wide selection of wavelengths, in order to be competitive in applications in which solid-state and other lasers are currently used, at least an order-of-magnitude, and preferably two orders-of-magnitude increase in power over that offered by prior-art OPS-lasers is required. This power increase must also be achieved without sacrifice of output-power stability and beam-quality. Further, in order to be applicable in the broadest range of applications the range of OPS-laser wavelengths available at high-power and with high beam-quality must be extended into the UV region of the electromagnetic spectrum, preferably to wavelengths less than 300 nm.
The present invention is directed to providing high-power OPS-lasers, including high-power OPS-lasers providing ultraviolet radiation, i.e., at wavelengths less than about 425 nm. In one particular aspect, an OPS-laser in accordance with the present invention comprises an OPS-structure having a gain-structure surmounting a mirror-structure. The gain-structure includes a plurality of active layers having pump-light-absorbing layers therebetween. The active layers have a composition selected to provide emission of electromagnetic radiation at a predetermined fundamental-wavelength between about 425 nanometers and 1800 nanometers when optical-pump light is incident on the gain-structure. The mirror-structure includes a plurality of layers of alternating high and low refractive index and having an optical thickness of about one-quarter wavelength of the predetermined wavelength.
A laser-resonator is formed between the mirror-structure of the OPS-structure and a reflector spaced apart therefrom. An optical arrangement is provided for delivering the pump-light to the gain-structure, thereby causing fundamental laser-radiation having the fundamental wavelength to oscillate in the laser-resonator. A heat-sink arrangement is provided for cooling the OPS-structure. An optically-nonlinear crystal is located in the laser-resonator and arranged for frequency-doubling the fundamental laser-radiation, thereby providing frequency-doubled radiation having a wavelength half of the fundamental wavelength.
The laser-resonator, the optically nonlinear-crystal, the OPS-structure, the heat-sink arrangement and the optical pump-light-delivering arrangement are selected and arranged such that the resonator delivers the frequency-doubled radiation as output-radiation having a wavelength between about 212 nanometers and 900 nanometers at an output-power greater than about 100 milliwatts. The laser preferably has a resonator length greater than about 5.0 cm
In one embodiment of a high-power OPS-laser in accordance with the present invention, stable, single axial-mode, CW laser output-power of about 4.0 W at 488 nm wavelength is achieved by intracavity frequency-doubling 976 nm radiation from a single OPS-structure using an optically-nonlinear crystal of lithium triborate (LBO) in a resonator having a length of about twenty-five centimeters (cm). The OPS-structure has active layers of an In0.18Ga0.82As composition, and pump-light-absorbing (separator) layers of a GaAs0.978P0.022 composition. The laser is pumped by about 34.0 W of 808 nm radiation from two diode-laser arrays. Numerical models indicate that the same resonator may be modified by including an optically-nonlinear crystal for mixing the fundamental and frequency-doubled radiation to produce about 120.0 mW of (frequency tripled or third-harmonic) 325 nm UV radiation. A simplified configuration of the resonator of this example was used without an optically nonlinear crystal to deliver fundamental output-power of about 10 W.
The 4.0 W of frequency-doubled output-power represents over two orders-of-magnitude increase over what is believed to be the highest reported frequency-doubled output-power of any prior-art OPS-laser. It is believed that frequency-tripled output of any power has not been achieved in a prior-art OPS-laser.
Numerical models indicate that in a resonator similar to the resonator of the example above, an OPS-structure having a gain-structure InxGa1-xP quantum wells with InyGa1-yAszP1-z separator layers therebetween, wherein x is selected to provide fundamental radiation at 750 nm may be pumped by 12.0 Watts of pump-light at 670 nm, using a 5 mm-long LBO crystal for frequency doubling to provide output-power in excess of 1 Watt at the frequency-doubled wavelength of 375 nm.
This remarkable increase in OPS-laser output-power and the ability to generate high, CW, UV output-power, either by frequency-doubling or frequency-tripling, is achieved without sacrifice of beam-quality. Single mode operation provides that OPS-lasers in accordance with the present invention can have a beam quality less than 2.0 times, and as low as 1.2 times the diffraction limit. This high-beam quality makes the inventive OPS-lasers ideal for applications in which the output radiation must be focused to a very small spot for making precise incisions in inorganic or organic material, or must be efficiently coupled into an optical fiber for transport to a location where it is to be used.
In another aspect of an OPS-laser in accordance with the present invention, the laser includes first and second resonators arranged such that a portion of the resonator axes of each are on a coaxial path. The first resonator includes an OPS-structure arranged outside the coaxial path to provide a selected fundamental-wavelength of laser radiation. Located on the coaxial path of the first and second resonators is an optically-nonlinear crystal arranged for frequency-doubling the fundamental radiation. The first and second resonators are interferometrically matched to maintain optimum phase-matching between fundamental and frequency-doubled radiation in the optically-nonlinear crystal. Fundamental-wavelength radiation and frequency-doubled radiation circulate together only along the coaxial path. An optically-nonlinear crystal is located in the second resonator outside the coaxial path for doubling the frequency of the frequency-doubled radiation thereby providing frequency-quadrupled radiation. Numerical models indicate that by using the 976 nm OPS-structure and pumping arrangement of the above-described first embodiment, this second embodiment is capable of providing about 2.0 W of frequency-quadrupled (244 nm) radiation.
In yet another aspect of an OPS-laser in accordance with the present invention, the laser includes first and second interferometrically-matched resonators arranged such that a portion of the resonator axes are on a coaxial path. The coaxial path includes a first optically-nonlinear crystal arranged for frequency-doubling as discussed above with respect to the second embodiment. Fundamental-wavelength radiation and frequency-doubled radiation circulate together only along the coaxial path. A second optically-nonlinear crystal is located in the coaxial path of the first and second resonators for mixing the fundamental radiation with frequency-doubled radiation thereby providing frequency-tripled radiation. Numerical models indicate that by using the 976 nm OPS-structure and pumping arrangement of the above-described first embodiment, this third embodiment is capable of providing about 4.0 W of frequency-tripled (325 nm) radiation.
Other, more general, aspects of OPS-lasers in accordance with the present invention include but are not limited to: design of heat-sink configurations and bonding methods for cooling an OPS-structure which allow the above exemplified high pump-powers to be directed on the OPS-structure while maintaining a safe operating temperature therefor; design of optically-long resonators for providing a relatively large fundamental mode-size at OPS-structure to take advantage of a larger pumped-area, thereby increasing laser output-power; design of specific, folded-resonator configurations for optimizing output of frequency-converted radiation and preventing reflection of the frequency-converted radiation back into the OPS-structure where it would be lost through absorption; selection of a specific ratio of pumped area to mode-size at the OPS to optimize use of gain, and to prevent generation of transverse modes of oscillation; use of an intracavity wavelength-selective element for preventing oscillation of fundamental radiation at wavelengths outside the spectral range of acceptance of optically nonlinear crystals; selection of optically nonlinear materials for maximum spectral acceptance to allow the use of efficient and tolerant wavelength-selective devices for the former; configuration of OPS-structures to eliminate parasitic lateral oscillation which would otherwise reduce output power; design of OPS-structures for minimum net stress and reliability under high power operation; use of radial-index gradient lens to optimize multiple optical-fiber delivery of pump-light; and design of mirror-structures for the inventive OPS-structure for maximum thermal-conductivity thereby facilitating cooling of the OPS-structures.
It will be particularly evident from the detailed description of the present invention presented below that for achieving the high powers discussed above, OPS-laser resonators in accordance with the present invention depart radically from the xe2x80x9ccompactnessxe2x80x9d philosophy of prior-art OPS-lasers and are inventively configured for intracavity frequency multiplication. It will also be evident that significant attention is directed to thermal management of OPS-structures, to the design of OPS-structures themselves, and to selection of frequency multiplication materials, in order to achieve the remarkable output-power levels, and stability of the inventive OPS-lasers. It will further be evident that certain inventive aspects of the invention are applicable to both high and low-power OPS-lasers, or even to laser types other than OPS-lasers.